The present invention relates to apparatus for reproducing data stored on digital-signal recorded optical disks, and particularly to a disk reproducing apparatus suitable for high-speed access to disks on which data is recorded at a constant linear velocity.
Recently, compact disk players (CD players) have been widely used as music players. The optical pickup of the CD player detects the digital data of an audio signal recorded on a compact disk. This reproduced digital data is subjected to error detection and correction processing and so on, and then is converted into the original analog audio signal. This CD player is able to perform high-speed access to the disk because the optical pickup is contactless with respect to the disk surface. In addition, the CD player is able to easily search for a desired piece of music because address information is also recorded on the disk. This compact disk (CD), when rotated, has different angular velocities at the inner and outer peripheries; in this regard the angular velocity at the outer periphery is about twice that at the inner periphery. Even if the compact disk has concentric tracks or a spiral track, it can record data at a uniform density over its entire surface. In the compact disk, data is recorded using a constant linear velocity (hereinafter, referred to as CLV) system by which the recording density can be increased as compared with a constant angular velocity (hereinafter, referred to as CAV) system. Upon reproduction, the compact disk is also controlled to rotate at a CLV as described in JP-A-59-185071.
A CD-ROM (compact disc-read-only memory) is known which is used as a CD for a data memory. When the CD-ROM used as a data memory is accessed for reading, it is necessary to reproduce (or read) recorded data at a high access speed. One of the factors for determining this access speed is the revolution rate control for the disk. Since the CD is recorded (or written) in the CLV system according to a standard, rotating this CD under CLV control upon reproduction will cause the problem that the revolution rate at the outer periphery is about twice that at the inner periphery on the CD.
Thus, if the CD is rotated under CLV control upon reproduction, the time in which the linear velocity is controlled to be constant, or the overhead time, upon access to the outer periphery from the inner periphery or to the inner periphery from the outer periphery, becomes too large to be neglected for increasing the reproduction access speed. The time in which the linear velocity is controlled to be constant depends on the performance of a drive motor for rotating the disk. So far, attempts have been made to increase access speed by improving the performance of the motor, but such increase of the access speed is limited.
JP-A-64-60863 (NEC) discloses a data recording technique for recording data using a constant linear velocity system, to increase the recording density on the disk in a recorder of the type in which the disk is rotated at a constant revolution rate, or in the CAV system recorder. In the JP-A-64-60863, however, no specific countermeasures necessary upon reproduction are presented except for features relating to the recording method.